1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus capable of generating ultrasound images of a high spatial resolution and a high contrast resolution, and more particularly, to an ultrasound diagnosis apparatus capable of transmitting and receiving transmission beams and reception beams, each having substantially uniform thin beam width along an elevation direction, from and to ultrasound transducers.
2. Description of the Related Art
An ultrasound diagnosis system transmits ultrasound pulses from ultrasound transducers (hereinafter “transducers”) installed in a head portion of the ultrasound probe to an object, such as a patient. The transducers receive reflected (echo) ultrasounds that are generated in accordance with differences of acoustic impedances of organs in the object in order to display the organ images on a monitor. Since an ultrasound image diagnosis apparatus can easily obtain and display a two dimensional image or a three dimensional image of B mode data or color Doppler data in real time by simply touching an ultrasound probe to a patient's body surface, it is widely used as an apparatus for diagnosing the status of a target organ in a patient's body.
A recent ultrasound diagnosis apparatus can electronically control ultrasound transmission/reception directions and focusing points of the transmission/reception waves in order to improve a spatial resolution and a high contrast resolution of image data. Doing so, the ultrasound transmission beams are focused by controlling delay times for the respective transmitting drive signals supplied to each of a plurality of transducers in an array, and the ultrasound reception (echo) signals acquired through a plurality of transducers are also focused by giving the respective delay times to each of the echo signals with performing receiving phase compensation for focusing and summation (hereinafter, simply referred to as “phase compensation and summation” or “phase compensation/summation”).
When a plurality of receiving signals acquired through the plurality of reception transducers are focused by performing the phase compensation and summation to each of the receiving signals, the respective reception timing of the echo signals reflected in the object are depended on an elevation (depth) distance from the transducer surfaces to the reflection body. Consequently, a particular method has been developed to form the focused reception beams along a depth (elevation) direction in the object by successively renewing the delay time that is applied to the reception signals acquired in time series through a plurality of reception transducers (hereinafter, “dynamic focusing method”).
When a plurality of transmission beams are focused by controlling each delay time for the transmission driving signals, the transmitting ultrasounds emitted from each of the plurality of transducers propagate into the object to a prescribed transmission focusing point based on transmission wave-fronts determined by the delay times for the driving signals. FIG. 6A illustrates a focusing status of the transmission beams by controlling the delay times for transmission driving signals. Thus, the focusing area of the transmission beams is limited on and around a transmission focusing distance (20 mm point in FIG. 6A). However, the transmission beams apart from the focusing point have wide beam widths along an azimuth direction at the shallower area (10 mm point in FIG. 6A) or the deeper area (30 mm point in FIG. 6A) than the focusing area along elevation direction.
The spatial resolution and the contrast resolution of ultrasound images data are highly dependent on the transmitting beam width and the receiving beam width. Consequently, when the transmitting beam widths are substantially different at the depths, remarkable differences occur between the quality of the image data generated around the area of the transmission focusing point and the quality of the image data generated at the distances apart from the transmission focusing point.
Typically, when a strong beam focusing is executed by using a transducers group having a large width used for a transmission (i.e. a large transmission aperture), a conspicuous deterioration of image data quality occurs at the areas apart from the transmission focusing point. On the contrary, when a weak beam focusing is executed by using a small transmission aperture, it becomes impossible to narrow a beam width for a transmission focusing area. A signal to noise ratio (S/N) of image data also deteriorates in company with a reduction of the transmission power.
To measure these problems, Japanese patent application publication 2007-323029 has proposed an improved method for generating image data of a good quality by setting a plurality of transmission focusing points along an elevation direction for repeated ultrasound transmissions/receptions along with successively renewing the transmission focusing point in order to extract the reception signals that are acquired from each of the transmission focusing points and the vicinity areas only. Hereinafter, this method is referred as a “multi-stages focusing method”.
However, the multi-stages focusing method needs to repeat ultrasound transmissions many times to the different transmission focusing points in the same direction in order to form transmission beams having a substantially uniform thin beam width along an elevation direction. Accordingly, the proposed multi-stages focusing method has a problem in that a time resolution (frame rate) for acquiring the image data is remarkably deteriorated. Further, the proposed multi-stages focusing method has another problem in that the transmission energy emitted into the object can not be effectively used since the receiving signals are acquired at areas other than the transmission focusing area.